Two alternatives are generally used for providing the ledger support at the roof line (and in some cases floor lines) around the inside perimeter of buildings and/or at internal concrete or masonry walls. These are either wood ledgers such as 4.times.14's or metal ledgers such as angle or U-shaped, both attached to the concrete or masonry wall by means of anchor bolts.
The wood ledger has historically dominated until recent years, primarily due to the ease of attachment it provides for intersecting plywood, purlins, and other structural members attached to the ledger. More recently the wood ledger systems have been under attack because of their excessive costs, certain deficiencies in respect to seismic tie anchorage, and the practical difficulties encountered in making the concrete to outstanding member connection without an intermediate wood connection, as now required by seismic codes.
Several years ago, the metal ledger was rapidly making its appearance, typified by 3.times.3.times.1/4 inch up through 3.times.6.times.1/4 inch angled sections, as well as U-channel sections of approximately the same net section. In part, this development was made practical by the improvement in gun, air driven or several drilled screw methods for attaching plywood, hangers, and other hardware to the metal ledger.
About three (3) years ago, the metal ledger system received a bad set back when a series of tests disclosed that when a typical purlin type hanger was attached to such an angle or U-channel, the loading radically deflected the outstanding leg of the angle or U-channel at the points of concentrated loading. Therefore, in order to use metal ledger angle or U-sections, it became necessary to provide costly field welding of gussets (metal plates) at the point of the concentrated loads to prevent deflection of the outstanding leg of the ledger member. This requirement tended to throw the balance of economic value back towards use of the wood ledger.
More recently, the economic balance has shifted back to the use of the metal ledgers by virtue of the fact that the net section of the metal ledgers may be calculated, approximately, in lieu of an equal section of reinforcing steel in the concrete or masonry walls otherwise required as bond steel in such structures. This economic factor at present is sufficient to overcome the cost of field welding gussets so that there is a sudden surge of metal ledger designs being specified.